A deal on gun control in the US took a significant step forward on Wednesday when two senators announced a bipartisan compromise that would amount to the first substantive action in Congress since the shootings at Sandy Hook elementary school last year.

The carefully-worded deal, struck by senators Pat Toomey and Joe Manchin, falls far short of the crackdown on assault weapons sought by the White House, but seeks to keep Republicans on board in ways that give it a chance of becoming law.

Under the proposal, sales of weapons at gun shows and online would require the same federal screening of criminals and the mental ill as currently apply in gun shops. Private trades would remain exempt.

The two senators also said they would simultaneously seek to strengthen protection for lawful gun owners by allowing them to carry weapons more easily across state lines. It would also make it illegal to use data from background checks to establish a national register of gun owners – a major fear of gun rights activists.

Announcing the deal, senator Pat Toomey, a Republican from Pennsylvania, said he had spoken to colleagues in the House of Representatives and "a substantial number were supportive of the general approach". He said the National Rifle Association had also been consulted during their talks.

In a break from its recent intransigence, the NRA called the deal a "positive development", though stressed that more needed to be done to address deficiencies in the mental health system. "Expanding background checks at gun shows will not prevent the next shooting," added the NRA in its statement.

In a nod to the NRA's position, Toomey also called for a national commission to study the root causes of violence in society. "I am a gun owner and the rights that are embedded in the second amendment are very important to me," he added. "But I don't consider comprehensive background checks to be gun control; I think it is just common sense."

A spokeswoman for the Sandy Hook Promise, a campaign group set up by Newtown residents after December's shooting, said it had not yet received a briefing on the senate proposals but would respond when it saw them.

Families of Sandy Hook victims have been lobbying members of Congress in Washington this week, and Michelle Obama is expected to up the pressure with a rare foray into politics at a gun control event in Chicago later on Wednesday.

Manchin rejected criticism that such a deal would be too incremental to make much difference. "To get anything of this proportion would be amazing," he told reporters following a press conference at the Senate.

"This is just a start and not the end of our work," he added. "This will not take away the pain of events like Sandy Hook, but it has changed our hearts and minds."

At the start of the New Year, when President Obama announced he would use the public anger generated by the Sandy Hook massacre to push ahead with sweeping gun reform, comprehensive background checks were seen as the bare minimum for what Congress might approve. But fierce lobbying by the National Rifle Association has succeeded in making both Republicans and key Democrats wary of supporting any change in recent weeks.

The Toomey-Manchin deal still faces a series of obstacles, starting on Thursday when Republican opponents in the Senate may attempt to stop it even reaching the floor for a debate.

The senate proposal will be the first amendment to a new bill on gun control that majority leader Harry Reid will try to push forward with a motion to prevent opponents from using a filibuster.

But significantly, it appeared that Senate opponents such as minority leader Mitch McConnell were losing their battle to prevent the bill from being voted on. The Washington Post reported that a sufficient number of moderate Republicans had agreed to prevent conservative opponents from resorting to filibuster tactics to block a vote.

A bigger challenge will come in the House of Representatives where Tea-Party Republicans have sworn to block any measure that restricts the rights of gun owners.

Describing the proposed legislation, Toomey said: "The Public Safety And Second Amendment Rights Protection Act would require states and the federal government to send all necessary records on criminals and the violently mentally ill to the National Instant Criminal Background Check System. The bill extends the existing background check system to gun shows and online sales."